Gordon Brown is likely to face fresh questions over British military resources in Afghanistan today after a senior government minister admitted that troops lacked enough helicopters as it was disclosed that another British soldier had been killed.

The Foreign Office minister Lord Malloch-Brown, who is leaving the government at the end of this week, also admitted the public had been inadequately prepared for the US and British offensive in Helmand before the recent rise in casualties.

Adding to Brown's discomfort, Malloch-Brown conceded that the prime minister's future looked "bleak", while also casting doubt on the future of Britain's Trident nuclear deterrent.

The prime minister will come under pressure over the minister's remarks when he holds his final press conference before the summer recess in Downing Street today.

Lord Malloch-Brown's intervention in the row over the lack of helicopters is particularly damaging for the government because his role as Foreign Office minister includes responsibility for Afghanistan.

His comments came as it was confirmed that the 18th British soldier this month had been killed in Afghanistan. Captain Daniel Shepherd, 28, a bomb disposal expert from Lincoln, was killed as he defused a device while on patrol in central Helmand on Monday. A second soldier was injured in the blast.

Defence chiefs asked weeks ago for more troops and have expressed concern about the lack of helicopters for some time. General Sir Richard Dannatt, the retiring head of the army, said he had "no regrets" at speaking out publicly about soldiers' needs.

"There is a line which generals speaking publicly should not cross ... I don't believe I crossed it. We may have got quite close, but I will look back over my shoulder with no regrets at three years as chief of the general staff."

Malloch-Brown also spoke frankly today about the shortage of equipment, providing political cover for Dannatt.

"We definitely don't have enough helicopters. When you have these modern operations and insurgent strikes what you need, above all else, is mobility," he told the Daily Telegraph.

The minister went on to admit that the public had not been prepared for the intense fighting in Helmand, a stronghold of the Taliban in southern Afghanistan.

"We didn't do a good job a month ago of warning the British public that we and the Americans were going on the offensive in Helmand," the peer said. "This is a new operation; the whole purpose is to win control. These deaths have happened ... after we chose to go on the offensive."

Lord Malloch-Brown also controversially suggested that the Taliban may have to contribute to a future Afghan government for there to be peace in the region.

Elements of the insurgents' "support group" may have to be invited back into "the political settlement" as a price of victory, he said.

Professor Michael Clarke, director of the defence thinktank the Royal United Services Institute, said Lord Malloch-Brown's comments were an "astonishing" challenge to the government to rethink its Afghanistan strategy.

He told BBC Radio 4's Today programme the row over helicopters had assumed a "totemic" significance.

He said: "Everyone agrees it would be better if there were more lift helicopters ... in Afghanistan because they give you the flexibility to move people around.

"But on their own, helicopters are no silver bullet for winning wars."

Clarke added: "It is astonishing to me that Malloch-Brown has said this before he steps down from the government because he seems to be throwing down a challenge, which is to say 'we have to rethink our strategic priorities over Afghanistan and what we are trying to achieve there'.

"That is something a number of people have said, but for a government minister to say this at this time is very interesting."

The chancellor, Alistair Darling, also stepped into the debate over armed forces equipment levels, saying in an interview with Tribune magazine that he had funded all requests from the military.

"The army has said this is what we want in terms of troops and equipment, and we have provided that and financed it ... In the face of acute danger in somewhere like Afghanistan, you have to make sure there are sufficient troops and that those troops are sufficiently equipped to do what is asked of them."